Bipolar transistors are generally composed by two adjacent pn-junctions lying close together in a semiconductor crystal. In different configurations, either two n-doped regions are separated by a p-doped region (npn-type transistor) or, alternatively, two p-doped regions by an n-doped region (pnp-type transistor). The three doped regions are generally referred to as emitter, base, and collector. Therefore, a bipolar transistor is a three terminal device having three doped regions of alternating doping type.
Bipolar transistors may exhibit desirable properties such as high current gain and high cut-off frequency for switching applications or microwave (RF) amplifier applications. These properties make bipolar transistors important components in logic circuits, communications systems, and microwave devices. As with other types of semiconductor devices, there is an increasing demand for bipolar transistors having high operating frequencies and/or switching speeds. Since their invention in 1947, many attempts have been made to meet these demands and improve the performance of such transistors with respect to their speed, power, and frequency characteristics. These attempts have focused on making devices better suited for high frequency applications such as microwave and logic devices. One particular way to meet these demands for transistors with high cut-off frequencies and high current gain is to reduce external base resistance (also referred to as base link resistance) and a low leakage current. Leakage currents may occur, when dislocations diffuse from the interface between external base contact layer (e.g. polycrystalline silicon) and internal base region (monocrystalline silicon) into the active portion of the base region. In order to avoid these diffusion of dislocations into the active portion of the base regions, the distance between the active portion of the base region and the mentioned interface should not be too short. Although, a further reduction of this distance would reduce the base link resistance this would also entail an undesired increase of leakage current. Thus, there is a conflict of objectives when designing the geometry of a bipolar transistor.